1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a laser printer or an ink jet printer, and more particularly to an image forming apparatus having a function of judging the type of sheet of a recording material by an image pickup device.
2. Related Background Art
Conventionally, the sheet type of a recording material used in the image forming apparatus of this type varies widely in thickness, basis weight, surface property, and material type. In the case where various recording materials are used to form an image under the same process conditions, there arises a problem where, although an excellent image is outputted on a recording material of a certain sort, the excellent image cannot be obtained on recording materials of other sorts.
In particular, with respect to the surface property of the recording material, the process conditions under which the output image becomes optimal (for example, a fixing temperature of an electrophotographic apparatus using a thermal fixing system) are largely different depending on whether the recording material has a smooth surface or the recording material has a rough surface (rough paper). Thus, a large difference in image quality occurs in the case where the image is outputted on different recording materials under the same process conditions.
For that reason, it has been proposed that a picture of a surface of the recording material is read by an image pickup device such as a CCD, the surface property of the recording material is detected by a given algorithm on the basis of the picked-up image, and the process conditions are automatically adjusted in accordance with the detected result as disclosed in, for example, Japanese Patent Application Laid-Open No. 11-271037.
However, the above-mentioned conventional image forming apparatus suffers from the following problems.
That is, in a structure where an image pickup device is disposed on a transporting path as disclosed in Japanese Patent Application Laid-Open No. 11-271037, the picture of the surface of the recording material is read while the recording material is being transported, and therefore it is necessary to set the shutter speed of the image pickup device high in order to obtain an excellent read picture without any blurring. Therefore, it becomes necessary to employ an image pickup device which is higher in performance and cost, and to increase the quantity of light which is irradiated onto the surface of the recording material since the shutter speed is made high. As a result, a user must bear the higher costs.
Also, in a structure where the image pickup device is disposed in a sheet feeding part, there is disadvantageous in that only the sheet type of a recording material on a specific sheet feeding part can be detected in the case of an image forming apparatus having a plurality of sheet feeding parts, or the image pickup devices of the same number as that of the sheet feeding parts needs to be disposed. Likewise, the user must bear the high costs in this case.